1. Field of the Invention
The present invention relates to an exhaust emission control apparatus for controlling an exhaust emission or gas of a general-purpose internal combustion engine by applying a secondary air to a ternary catalyst provided in an exhaust passage of the engine.
2. Description of the Related Art
Technology is known in which for controlling an exhaust emission of a general-purpose internal combustion engine, a secondary air is applied to a ternary catalyst provided in an exhaust passage of the engine to thereby improve the rate of oxidization of pollutants in the exhaust emission.
Such known technology is disclosed in, for example, Japanese Patent Laid-Open Publication Nos. HEI-4-325709 entitled "METHOD AND APPARATUS FOR CONTROLLING AN EXHAUST EMISSION OF AN INTERNAL COMBUSTION ENGINE" and HEI-5-113119 entitled "GENERAL-PURPOSE INTERNAL COMBUSTION ENGINE HAVING A SECONDARY AIR SUPPLY DEVICE".
The exhaust emission control apparatus disclosed in Japanese Patent Laid-Open Publication No. HEI-4-325709 has a catalytic member disposed closely to an outlet of a muffler and is arranged such that the atmosphere is caused to come into a downstream portion of the catalytic member by exhaust pulsation, whereby the oxidization reaction of the exhaust emission at the downstream portion of the catalytic member is promoted by the backflown air.
However, the disclosed apparatus has a drawback in that the amount of the backflown air is small and hence the apparatus can hardly pass a required high exhaust gas control value.
In order to achieve the pulsation more effectively, the general-purpose internal engine disclosed in Japanese Patent Laid-Open Publication No. HEI 5-113119 has a tailpipe which is longer than an exhaust pipe connected to a muffler with exhaust gas control capability and connected at an upstream end thereof to the muffler.
However, because the length of the tailpipe of the disclosed general-purpose internal combustion engine is set to provide optimal pulsation at a rated speed, high exhaust gas control efficiency can hardly be achieved in the engine operated at a wide range of engine speeds. Further, it is undesirable to have such a long tailpipe in the general-purpose internal combustion engine which is required to be compact.
Consequently, there is a demand for the provision of an exhaust emission control apparatus for a general-purpose internal combustion engine, which apparatus is compact and capable of supplying a sufficient secondary air and of coping with a wide range of engine speeds.